This invention relates to an apparatus for examining water quality, and more particularly to a water quality determination apparatus in which a sensor element is placed in contact with the water under investigation to electrochemically determine indications of representing the water quality of polluted rivers, lakes, marshes sewage and waste waters.
Of the water quality determination device of the kind described, an electrochemical type sensor element includes dissolved oxygen meters, PH meters, oxidation-reduction potentiometers, and other meters for determining concentrations of various ions by selective ion electrodes. An optical type sensor element includes immersed type turbidity meters and suspended solid concentration meters.
As a water resource has been contaminated, a treating and a monitoring techniques have established by way of preventation of the pollution of water resources. Various kind of measuring devices are used in putting the treating and supervising techniques to practical use. However, a first problem encountered in using these devices is that, as the objections of measurement are polluted rivers, lakes and marshes and sewage and waste waters, the sensing elements as placed in contact with the water to be examined have their own inherent problems such as the lowering of the sensitivity of the electrodes and the inability to measure acculately and continuously as the pollution of a surface of electrode has developed, because the objections of the measurement are sludges and polluted rivers and the like.
Taking the aforementioned dissolved oxygen meter by example, such meter employs an electromechanical reaction. A conventional oxygen meter principally comprises a negative electrode, a positive electrode, an electrolyte and a membrane for protecting these electrodes and the electrolyte from the test liquid. When the sensing element of the dissolved oxygen meter is placed in contact with the test liquid to be examined, gas molecules in the test liquid in the vicinity of the membrane of the sensing surface diffuse into the electrode through the membrane. Namely, the test water in the vicinity of the membrane will become the condition which is lack of the oxygen gas molecules in comparison with the other portions of the test water.
Accordingly, as is the case commonly with all the aforementioned types of dissolved oxygen meter, it is necessary to move the test water at a velocity higher than that of 10 to 50 centimeter per second relative to the sensor element to achieve a high accuracy measurement of the amount of dissolved oxygen by contacting the fresh test water to the membrane. Accordingly, the most significant problem in measuring the amount of dissolved oxygen meter is what may give the velocity of flow to the test liquid.
To work out this problem, when the test liquid is relatively clean, it is possible to give the relative speed between the test liquid and the sensing surface of the sensor which is positioned within a suitable conduit of a conventional mechanical pump.
However, when subjects of measurement are mixed liquid in the aeration tank of the sewage treating station, polluted rivers and lakes, the test liquid to be examined includes an unexpected shape and property matter such as hair, rubber and grindstone or the like, and these matters shorten the life of the mechanical pump. It has been found experimentally that increased effects may be oblained by using an air-lift pump which has a simple construction and has no rotary or frictional portion. Accordingly, as the air-lift pump is simple construction it is only required to remove the obstruction automatically in order to carry out the measurement long interval and continuously.
The second problem encountered in long interval and continuous measuring the dissolved oxygen in the polluted water is that when the dissolved oxygen electrode is immersed in the polluted test water, pollutants such as sludges stick to the surface of the sensing electrode. This means that the change of the effectual thickness of membrane, and especially in case a contamination is an aerobic microorganism which absorbs the effectual change of the thickness of the membrane will become greater and, therefore, the sensitivity of sensing electrode is lowered.
Hithertofore, although an ultrasonic wave, jet water and jet air have been used for the means for removing or preventing the pollutants, there are few cases in which the increased effect can be obtained. Namely, the method using the ultrasonic wave is only effective to remove the contaminate such as sands and sludges which is adhered to the sensor element, and is not effective to remove the cohesive matters consisting of the microorganism adhering to the sensing surface what may be called the sludges. And, further, in the ultrasonic wave cleaning type, there is a disadvantage that is necessary to use a costly ultrasonic wave generating device.
In the method using the jet water to clean the sensing element, as the high pressure water is blow against the sensing surface, it is impossible to clean entirely the sensing surface which is relatively wide surface and is curved shape. The more important disadvantages of the method used the high pressure jet water is the fact that a sensor element having not so mechanical strength is destroyed by the jet water, and costly pressure pump is required. And, furthermore, the more important disadvantages is the fact that the microorganism included in the sludge is destroyed by the jet water. And viewing the jet air means from a standpoint of maintenance free the method of jet water have the following drawbacks that since an air compressor is usually used for obtaining the high pressure air, it is necessary to check an oil, an high pressure tank of the air compressor, and further is required the costly and mechanically strong sensing element.
Refloating and watching a sensing device immersed in the test water during several days, a cohesive matter what may be called a pollutant has been adhered to the sensing surface of the device. A course of adhering and a rate of the cohesive matter is greatly depended upon the water quality and the condition of the flow of the test water. It may, however, be presumed that the pollutant is not chemically connected to components consisting of the sensor element judging from being separated from the sensing surface by wiping up with a piece of cloth or a paper. In fact, a sensitivity of the device recoveres and becomes normal conditions, therefore it may be judged that the pollutant is clinging to the sensing surface only physically. On the other hand, it is found that the adhering speed of the pollutant is reduced as the velocity of flow increases. And the sensitivity recovers to that of clean sensor. These phenomenon has been experimentally confirmed.